Immunity community

As I've noted previously, the human gut is the natural habitat for a large and diverse community of microbes that have adapted to live in its normal conditions and want to keep it that way.

the homeboys: a red blood cell, a platelet, and a white blood cell hang out
electron microscope image: The National Cancer Institute at Frederick

Biological fitness for bacteria is the same as it is for the rest of us: they need to survive and reproduce in order to pass on their genes.

They are single-celled and mainly reproduce asexually (they double their genetic material and divide in half, to form 2 identical cells), so each duplicate cell is equally adapted to its host's gut environment and doesn't want to see it messed up.

The resident bacteria also consume (break down) the nutrients, such as iron, that we provide, which leaves less for the alien microbes. Resident gut bacteria thus serve as committed players in our fight against infection and disease.

Butyrate in the gut changes "T" cells (a type of white blood cell that works in the immune system) into regulatory T cells. Regulatory T cells slow the immune response, which helps contain excessive inflammatory responses, such as autoimmune disorders.

Inflammation, the body's immune response to harmful stimuli (like a bee or wasp sting, but can be longer-term), has been associated with many chronic diseases. For example, inflammatory bowel disease (IBD), a gut-related autoimmune disease, is thought to result from changes to gut bacteria from either foods or antibiotics.

Treatment for IBD typically begins with anti-inflammatory drugs, though new options include fecal bacteriotherapy (yep, it's what you think – recall the poo-exchanging mice from the last post), designed to restore a healthy composition of gut bacteria, just like the mice did on their own.

A growing number of medical researchers are pointing to the gut as a source of the inflammation in other diseases as well. Our gut flora actually help maintain the lining of our gut by feeding it with fatty acids they produce as they break down fiber. If the right microbes are missing, that source of nutrients for the epithelial tissue lining the gut disappears, which may weaken it (a yet-unproven condition called "leaky gut") and allow bacteria, toxins, and proteins to escape out into the bloodstream. Unpleasant, at best, and possibly linked to disease.

Evidence suggests that gut bacteria that escape our digestive tract may even trigger depression, which brings us to our recent, but increasing, evidence that gut bacteria may influence our mental states relating to eating and other activities.

Playing with your mind

Apparently, gut bacteria can not only recognize the hormones of their host (us, in this case), but also synthesize them. The microbial community down under can respond to both the nutrients that we've eaten and the state of some of our hormones.

Acting together, they communicate with part of our nervous system that works in the gastrointestinal tract and may even be able to influence our appetite.

Those nerve cells together used to be called the enteric nervous system but are now increasingly being given the far more elegant name "gut brain". Our gut brain is actually linked to our regular "Big" brain, and it very regularly tells our Big Brain when it is hungry. And when it is full.

Normally, when sufficient food is in our digestive tract, or nutrients have been moved from the intestines into the bloodstream, the gut brain signals to the Big Brain (hello, up there!) that it's full and we should stop eating. Right.

As this article published in Science magazine explains, eating too much fat may cause people to have to eat more and fattier food to feel the same satisfaction normally gained from eating a meal.

A long-term, high-fat diet changes how the gut signals the brain to both enjoy and to stop eating. The change suppresses the production of dopamine in the brain, which, in turn, leads to the need to eat more high-fat foods to get that same satisfaction from eating.

The researchers suggest that restoring proper signaling from the gut to the brain (which they did by administering a certain gut-produced signaling molecule called oleoylethanoldamide) may make people feel satisfied and happier eating healthier foods.

The signals from gut to brain can be changed directly and predictably. Passive scaredy-cat mice that were fed gut bacteria from aggressive mice became more daring and aggressive, and vice versa: aggressive mice fed bacteria from passive mice became more passive.

My previous post on how eating certain high-fat, high-cholesterol foods — including eggs and beef — encourages bacteria to produce a compound, called TMAO, that seems to boost the risk of heart disease.

Site disclaimer: I’m not a medical doctor, dietitian, physical therapist, or personal trainer. This blog is for informational purposes only! I research topics from multiple sources, but can't guarantee the completeness or accuracy of every source, so please do your research regarding your own health and fitness.